Method for extraction heat from an effluent, especially waste water, circulating in a conduit, heat exchanger and material for implementing said method

ABSTRACT

The invention relates to a method for extracting heat from an effluent ( 2 ) circulating in a conduit ( 1 ), especially a waste water collector, according to which a heat exchanger (E) is installed, at least in the bottom of the conduit, said heat exchanger (E) lying in the effluent and being formed by coating tubes ( 3 ) with sufficiently heat-conductive concrete cast around the tubes intended for the circulation of a heat-transfer fluid, the heat exchange with the effluent of the conduit being carried out through the moulded coating. The concrete ( 4 ) of the coating consists of a least 50 weigh % of silicon carbide, a load of needles made of a heat-conductive and mechanically resistant material, a binding agent and the remainder of alumina, metal powder or carbon.

The invention relates to a method for extracting heat from an effluent flowing along a pipe, notably a sewer, according to which method there is installed, at least in the bottom of the pipe, a heat exchanger which is immersed in the effluent, this hear exchanger being formed by coating tubes with a sufficiently thermally conductive concrete poured around tubes through which a heat-transfer fluid is intended to circulate, the exchange of heat with the effluent of the pipe taking place through the cast coating.

Sewers or pipes for removing waste water carry dirty water which is warm or temperate as a result, of its residential or tertiary origins, or as a result of it originating from communal or industrial activity. Its temperature is generally comprised between 15 and 20° C.

The sensible heat of this water represents a source of energy that can be recuperated for the purposes of heating buildings, producing domestic hot water, or any other thermal use, in combination with a heat pump.

DE 35 21 285 and BP 1 215 460 disclose water pipes used with a heat exchanger embedded in the concrete wail of the pipe. Such exchangers have an attractive cost price, from the viewpoint of manufacture and implementation, compared with heat exchangers made from metal components. However, the transmission of heat through the concrete needs to be improved, as does the mechanical strength of this exchanger which may have to withstand maintenance personnel moving around on it.

It is an especial object of the invention to propose a method that makes it possible to obtain an exchanger of the kind concerned in which the thermally conductive effect and the robustness of the exchanger are simultaneously improved.

According to the invention, the method for extracting heat from an effluent flowing along a pipe, notably a sewer, of the kind defined hereinabove, is characterized in that the coating concrete is made up of at least 50% by weight of silicon carbide, of an acicular filler or needles of a thermally conductive and mechanically strong material, of a binder, the rest being alumina, metallic powder or carbon.

The acicular filler of needles preferably represents more than 2% by weight. The needles may be made of metal, particularly of carbon steel or aluminum, or may be made of carbon. The diameter of the needles is preferably less than five-tenth of a millimeter, and the length of the needles is preferably less than 10 mm.

Advantageously, a layer of thermally insulating material is placed between the tubes and the wail of the pipe, a layer of conductive concrete is placed in contact with the tubes, between the tubes and the effluent; and, at the surface, a layer of abrasion-resistant material is placed in contact with the effluent.

An intermediate mesh made of metal or synthetic fibers may be spread over the tubes, prior to the pouring of the coating material, over the entire extent of a portion of the exchanger so as to enhance mechanical strength and/or improve heat transfer.

The tubes may be ringed. Advantageously, the tubes are made of plastic. These tubes are preferably flexible.

The invention also relates to an exchanger for extracting heat from an effluent flowing along a pipe, notably a sewer, installed at least in the bottom of the pipe so that it is immersed in the effluent, the exchanger being made up of tubes embedded in a thermally conductive concrete poured around tubes through which a heat-transfer fluid is intended to circulate, the exchange of heat with the effluent of the pipe taking place through the cast coating, characterised in that the coating concrete is made up of at least 50% by weight of silicon carbide, of an acicular filler of needles of a thermally conductive and mechanically strong material, of a binder, the rest being alumina, metallic powder or carbon.

For preference, the tubes embedded in the concrete are ringed tubes. The tubes embedded in the concrete may be made of plastic. These tubes may be flexible.

Advantageously, the acicular filler of needles in the concrete represents at least 2% by mass, and the needles are preferably metallic needles, particularly made of carbon steel or of aluminum, or of carbon. In general, the binder consists of cement.

The exchanger is preferably produced, according to the method defined hereinabove.

The invention also relates to a material for implementing the method defined hereinabove, this material being characterized in that it consists of a mixture made up of at least 50% by weight of silicon carbide, of an acicular filler of needles of a thermally conductive and mechanically strong material, of a binder, the rest being alumina, metallic powder or carbon.

Advantageously, the acicular filler of needles represents at least 2% by weight. The needles may be metallic, particularly made of carbon steel or of aluminum, or may be made of carbon.

The thermal conductivity of the coating material according to the invention is significantly improved by the silicon carbide which has high thermal conductivity. Silicon carbide, combined with alumina and with a binder leads to a type of concrete that is easy to work.

The coating concrete according to the invention consists of a combination that makes it possible to obtain a composite that exhibits cohesion and mechanical integrity. The thermal conductivity of the alumina, which is higher than that of a conventional, concrete, makes it possible to obtain a composite of a conductivity that is higher than that of a mixture using conventional concrete.

The acicular filler of needles added to the combination of silicon carbide and alumina makes it possible to improve the mechanical integrity of the alumina/silicon-carbide mixture, and also to improve its thermal conductivity by creating thermal bridges that encourage the transmission of thermal flux.

The mechanical properties of the mixture obtained, and notably its high mechanical strength, allow it to be used in environments subjected to high stresses such as, for example, the passage of an operator wearing hobnail boots or a tool dropped from man height.

The invention consists, in addition to the provisions set forth hereabove, of a certain number of other provisions that will be discussed more explicitly hereinafter with regard to some entirely nonlimiting example embodiments described with reference to the attached drawing. In these drawings:

FIG. 1 is a cross section through a sewer equipped in its lower part with an exchanger according to the invention.

FIG. 2 is a cross section through a sewer, with a damaged lower part, prior to the installation of an exchanger according to the invention.

FIG. 3 is a partial section on a larger scale of an alternative form of embodiment; and

FIG. 4 is a schematic partial longitudinal section through a coated ringed tube of an exchanger according to the invention.

Reference is made to the drawings, notably to FIG. 1, which shows a pipe 1 forming a sewer for waste water 2 above the flow of which there is head space. Installed in the lower part of the pipe 1 is a heat exchanger E which is immersed in the effluent consisting of the waste water 2.

The exchanger E is cast and made up of layers of flexible supple tubes 3, preferably ringed or possibly smooth, which run parallel to one another and to the longitudinal direction of the pipe. The conductive concrete 4 is poured in situ over the tubes 3 and when it solidifies will perform a dual function of affording mechanical protection and allowing exchange of heat. The tubes 3 may be semi-rigid 3, made of metal.

The tubes 3 do not need to have intrinsic mechanical integrity; they need to allow the heat-transfer fluid 5 to circulate without the risk of leakage with optimized exchange of heat. In that respect, ringed tubes made of plastic (polyvinyl chloride PVC, polyethylene PE or the like) allow both an increase (by around 20%) in the surface area for heat exchange and, above all, an increase in the internal superficial heat exchange coefficient because of the turbulent flow conditions brought about by the profile of the ringed tubes and indicated, schematically in FIG. 4.

The concrete 4 with which the tubes are coated is made up of at least 50% by weight of silicon carbide, of an acicular filler of needles of a thermally conductive and mechanically strong material, of a binder, the rest being made up of alumina. The binder generally consists of cement.

The silicon carbide content may be as high as 90% by weight. The acicular filler of needles preferably represents at least 2% by weight.

The needles of the acicular filler are preferably made of metal, or of carbon. Metal needles are advantageously made of carbon steel or of aluminum.

The diameter of the needles is generally less than five-tenths of a millimeter for a length generally of less than 10 mm.

The thermal conductivity of the concrete according to the invention is considerably improved over that of a conventional concrete and, at the same time, the mechanical strength of the concrete, notably at the surface, is greatly improved by the presence of the needles.

The conductive concrete 4 according to the invention has excellent compression strength and resistance to erosion, good conductivity and an increased superficial heat exchange coefficient on the fluid side and on the waste water side.

The waste water heat exchanger is used in the lower part of the pipes and is covered by the flow, with head space, of waste water, according to the diagram of FIG. 1.

The heat exchange system is made up of two distinct parts:

-   -   a layer of a conductive mixture 4 of silicon carbide, alumina         and needles, notably ferritic needles, which line the bottom of         the pipe 1 and conform to the shape thereof,     -   an array of tubes 3, particularly ringed plastic tubes, trapped         within this layer of conductive material 4.

The energy contained in the waste water 2 is thus transferred to the heat-transfer fluid 5 circulating along the tubes 3, via the conductive material 4 and the tubes 3.

The use of ringed tubes 3 makes it possible not only to increase the area for exchange of heat between the conductive material 4 and the tubes 3, but also to create turbulence T (FIG. 4) in the heat-transfer fluid 5 circulating along the tubes 3, thus improving conditions of heat exchange on the internal wall, of the tubes 3.

The combination of the conductive concrete and of ringed tubes 3 embedded in the concrete makes it possible to obtain a good mechanical bond between the concrete and the tubes thanks to the succession of rings of the tubes 3, in addition, heat exchange is encouraged not only by the properties of the concrete but also by the ringed pipe effect which increases the exchange surface area and creates turbulent flow conditions for the heat-transfer fluid 5.

The heat exchanger 8 is divided into portions, in the direction in which the waste water flows, the size of which portions will be tailored such that the heat-transfer fluid 5 heats up by around 4° C., notably 4° C. to 8° C. For each portion of the exchanger, manifolds (not depicted) supply the tubes 3 with cold heat-transfer fluid. Having passed through the heat exchanger E, the warmed heat-transfer fluid is directed, via other manifolds (not depicted) to a heat pump. The heat-transfer fluid circulates in a closed circuit. The hydraulic balancing of the whole is advantageously performed using a Ticheimann loop.

The conductive concrete according to the invention can be used by spraying it onto the interior wall of the pipe 1, this considerably reducing the layering time and complexity of the operation, by comparison with a solution using a stainless steel exchanger.

The solution of the invention can be adapted to suit any geometry of pipe and can foe implemented by crews that have the skills needed for using a conventional concrete.

Because of the composition of the waste water, the pipes are subjected to stresses which lead to particularly pronounced wear in their submerged lower portions.

In addition to allowing the creation of a solution for exchanging heat with waste water, the new material according to the invention offers the advantage of allowing a pipe 1 a (FIG. 2) the lower part 6 of which has become damaged to be made good. Rather than replacing a worn pipe installing a heat exchanger E according to the invention in the lower part of the pipe la allows this pipe to be made good using the conductive concrete which will line the bottom of the pipe.

The high mechanical strength of the mixture protects the heat exchange system from any damage and makes it compatible with all the methods used for cleaning cut sewers.

To create the exchanger, the tubes 3 are first of ail fixed, by any suitable means, against the internal surface of the wail of the pipe 1. The coating concrete 4 was then poured, preferably sprayed, around the tubes 3 to harden in situ.

A protective film 6, notably a sheet of plastic, may be provided between the interior surface of the pipe 1 and the exchanger E incorporated into this pipe.

An intermediate mesh 7, notably made of metal or of synthetic fibers, is advantageously spread over the tubes 3, over the entire length of each portion of exchanger E in the direction in which waste water flows, so as to cover the entire layer of tubes 3. The mesh 7 enhances the mechanical strength. This mesh 7, when made of a thermally conductive material, notably of metal, contributes to improving heat transfer.

As illustrated according to the alternative form of FIG. 3, the tubes 3 may be coated in several layers of materials with different properties, namely:

-   -   a layer 8 of a thermally insulating material between the tubes 3         and the wall of the pipe 1 or between the tubes 3 and the film 6         when such a film is provided;     -   a layer of conductive concrete 4 according to the invert ion in         contact with the tubes, between the tubes 3 and the effluent,         and     -   at the surface, possibly a layer 9 of an erosion-resistant         material.

It should be noted that the increase in the mechanical strength of the conductive concrete means that the thickness of the layer of concrete between the tubes 3 and the waste water can be reduced, something that also encourages exchange of heat and combines with the improvement in thermal conductivity.

To quantify the physical properties of the cast exchanger E according to the operating parameters: calorific power, temperature and flowrate of the heat-transfer fluid and of the effluents, dimensions of pipes, profile in cross section, developed surface area, number and spacing of tubes, speed of fluids, etc., thermal and mechanical modeling will be carried out for each project.

Following commissioning, in-situ performance trials will allow the design parameters to be refined in order to optimise subsequent installations.

The exchanger is quick to implement, it requires no significant handling means.

-   -   The pipe 1 is emptied, cleaned out with a high-pressure jet         (possibly with sand added) and a keying coat of paint is sprayed         onto the surface that is to take the exchanger.     -   The layers of flexible tubes 3 wound onto a cylindrical reel are         introduced into the bottom of the pipe and fixed to racks that         space the tubes 3 cut.     -   After the layers of tubes 3 have been installed, a compressed         air or water leak test is performed to detect any leaks there         might be and these are then quickly repaired (connecting         sleeve).     -   The conductive concrete: cement e SiC+water+additive is prepared         outside the pipe (using a cement mixer or a concrete mixer         depending on the quantity).     -   The conductive cement prepared in a pasty (near-liquid) form is         poured over the tubes (using a concrete pump at the surface) and         then vibrated so that it fills all the cavities between and         rings of the tubes 3.     -   The manifolds for joining to the tubes situated at the ends of         the cast exchanger may be fitted before or after pouring.     -   To accelerate the hardening of the cement, an additive may be         incorporated at the time of preparation.

To increase the surface hardness it is also possible to incorporate a hardening resin into the mixture of the surface layer.

COMPARATIVE EXAMPLES

By comparison with a waste water heat exchanger made up of stainless steel modules, the heat exchange surface area needed with a conductive concrete according to the invention is approximately equivalent. However, the difficulty of implementation and the sewer downtime required for creating the exchanger are greatly reduced.

By comparison with a solution using conventional concrete with which to coat the tubes, the solution using the conductive concrete according to the invention requires an exchange surface area that is of the order of four times smaller.

Tests carried out using conductive concrete according to the invention, containing 90% by weight of silicon carbide (SiC), yielded a thermal conductivity of 72 W/m. °K. For a conventional concrete, the thermal conductivity is of the order of 1.4 W/m. °K. For a stainless steel exchanger, the thermal conductivity is of the order of 16 W/m. °K. The improvement afforded by the solution of the invention is therefore considerable.

The material of the invention (conductive mixture of silicon carbide, alumina and needles) can be used for any application that requires both heat transfer and good mechanical integrity.

The design of the cast exchanger of the invention meets the following main technical and economic criteria:

-   -   it maximizes the exchange of heat between the water flowing         along the waste water pipe and the heat-transfer fluid in the         closed circuit connected to the evaporator of the heat pump.     -   it minimizes the cost of manufacture and of implementation of         the device.

it makes the equipment more reliable, avoiding any risk of leakage and allowing the quick repair in the event of a localized accident.

-   -   it reduces the intervention time for in-situ installation.

The choice of tubes 3 may fail to ringed flexible tubes used in thermal applications for the ambient heating of greenhouses in the market-garden industry. Flexible underfloor heating tubes may also be used.

The excellent performance of the cast exchanger is dictated by the conductivity of the coating material and by the surface area for exchange between the heat-transfer tubes 3 and the material which is greater than that of a planar surface area because the heat flux passes by conduction through the entire surface area of the tubes, including their underside.

The mechanical strength of the material with which the tubes are coated means that its thickness can be limited, and it is possible to make repairs to the surface in areas which have suffered erosion after numerous years of use.

The surface of the cast exchanger in contact with the waste water preferably has a curved profile in cross section so that the velocity of the flow remains maximal when the water flowrate is low.

The exchange of heat across the non-submerged surface area of the cast exchanger will be significant since heat will be recovered from the saturated warm air present in the waste water pipe. The latent heat of condensation of the air increases the energy efficiency of the exchanger in periods of low waste water flowrate.

The reliability of the cast exchanger is of the same order of magnitude as the millions of m² of underfloor heating installed in the residential, tertiary and industrial sectors.

The cost of manufacture of a cast exchanger is very much lower than that of a mechanically assembled exchanger made up of stainless steel modules. The cost of implementation on site will be approximately halved, and the installation time will be likewise reduced.

The cost of maintenance will be minimum because the materials are rot-proof and repairs to the surface of the exchanger are envisioned after many years of use.

The environmental impact of construction and decommissioning is limited, because this technology may use materials which are essentially recycled: tubes, ground SiC, etc.

Combining ringed flexible tubes with an SiC-based conductive material makes it possible to produce a heat exchanger that is economical and at the same time offers good performance. 

1. A method for extracting heat from an effluent flowing along a pipe, notably a sewer, the method comprising installing, at least in the bottom of the pipe, a heat exchanger (E) which is immersed in the effluent the heat exchanger (E) being formed by coating tubes with a sufficiently thermally conductive concrete poured around tubes through which a heat-transfer fluid is intended to circulate, the exchange of heat with the effluent of the pipe taking place through the cast coating, wherein the coating concrete is made up of at least 50% by weight of silicon carbides of an acicular filler of needles of a thermally conductive and mechanically strong material, of a binder and the rest being alumina, metallic powder or carbon.
 2. The method as claimed in claim 1, wherein the acicular filler of needles represents more than 2% by weight.
 3. The method as claimed in claim 1, wherein, the needles of the acicular filler are made of metal, particularly of carbon steel or of aluminum, or are made of carbon.
 4. The method as claimed in claim 1, wherein the diameter of the needles is less than five-tenths of a millimeter for a length generally of less than 10 mm.
 5. The method as claimed in claim 1, wherein the needles are ferritic needles.
 6. The method as claimed in claim 1, wherein: a layer of thermally insulating material is placed between the tubes and a wall of the pipe, a layer of conductive concrete is placed in contact with the tubes, between the tubes and the effluent, and, at the surface, a layer of abrasion-resistant material is placed in contact with the effluent.
 7. The method as claimed in claim 1, wherein an intermediate mesh made of metal or synthetic fibers is spread over the tubes, prior to the pouring of the coating material, over the entire extent of a portion of the exchanger so as to enhance mechanical strength and/or improve heat transfer.
 8. The method as claimed in claim 1, wherein the tubes are ringed.
 9. The method as claimed in claim 1, wherein the tubes are made of plastic.
 10. The method as claimed in claim 1 wherein the tubes are flexible.
 11. An exchanger for extracting heat from an effluent flowing along a pipe, notably a sewer, installed at least in the bottom of the pipe so that it is immersed in the effluent, the exchanger comprising tubes embedded in a thermally conductive concrete poured around tubes through which a heat-transfer fluid is intended to circulate, the exchange of heat with the effluent of the pipe taking place through the cast coating, wherein the coating concrete is made up of at least 50% by weight of silicon carbide, of an acicular filler of needles of a thermally conductive and mechanically strong material, of a binder, the rest being alumina, metallic powder or carbon.
 12. The exchanger as claimed in claim 11, wherein the tubes embedded in the concrete are ringed tubes.
 13. The exchanger as claimed in claim 11, wherein the ringed tubes are made of plastic.
 14. The exchanger as claimed in any one of claim 11, wherein the tubes are flexible.
 15. A material for implementing a method as claimed in claim 1, the material comprising a mixture made up of at least 50% by weight of silicon carbide, of an acicular filler of needles of a thermally conductive and mechanically strong material, of a binder, the rest being alumina, metallic powder or carbon.
 16. The material as claimed in claim 15, wherein the acicular filler of needles represents at least 2% by weight.
 17. The material as claimed in claim 15, wherein and the acicular filler of needles are made of metal, particularly of carbon steel or of aluminum, or are made of carbon. 